The present invention relates to novel biologically-active vasopressin analogs. More particularly, the invention is concerned with vasopressin and its biologically-active polypeptide analogs which have been modified to produce long-acting forms of the polypeptides and are believed to function by slow release of an active vasopressin or vasopressin analog.
It has long been recognized that naturally formed vasopressin has biological effects, such as its antidiuretic activity and its vasoconstriction of visceral blood flow. Its utility, however, is limited by its relatively short half-life in the blood stream, as well as its well known general circulatory systemic pressor effect. Accordingly, a variety of analogs of vasopressin have been synthesized in an effort to modify the properties of vasopressin and provide products having increased pharmaceutical utility.
Efforts to extend the half-life of vasopressin involve modifications intended to inhibit enzymatic cleavage and consequent inactivation of the vasopressin molecule. For example, modification of vasopressin by deamination of cysteine at the 1 position and replacement of L-arginine by the D-isomer at postion 8 yields "desmopressin," which is immune to enzymatic cleavage of the 1-2 and 8-9 carbon-nitrogen bonds. As a consequence, desmopressin has been found to have enhanced and prolonged antidiuretic activity with low pressor activity. See U.S. Pat. No. 3,497,491. In a further modification, replacement of the disulfide bridge with a thioether linkage (--CH.sub.2 S-- or --SCH.sub.2 --) yields still other analogs having even greater potency and half-life, but without changing the biological target organs of the basic 1-desamino-vasopressin hormone. See U.S. Pat. No. 3,980,631.
Still another approach to new and potentially more effective vasopressin analogs is the preparation of hormonogen forms, i.e., vasopressin analogs having a chain of cleavable peptide residues attached to the active molecule. These analogs are thought to serve as reservoirs of the active molecule, which is slowly released by enzymatic cleavage of the added residues. This approach has so far involved a series of analogs obtained by attachment of one or more peptide residues to the N-terminal of the molecule, as is shown in U.S. Pat. No. 3,558,590. Of these analogs, N.sup..alpha. -(glycyl-glycyl-glycyl)-8-lysine-vasopressin, or tGLVP, is of principal interest. Although this modification has been effective in providing a longer-lasting agent, it still is not as effective as desired. For example, cleavage of the hormonogen also occurs at various sites in the active nonapeptide portion of the molecule, so that only a small fraction of the administered tGLVP is actually released in active form. As a result, large doses of tGLVP are required.
The N-terminal NH.sub.2 group is, however, not the only possible site of attachment of added residues to form a hormonogen. If a basic residue such as Lys, homoLys or Orn is used at amino acid position 8, the resulting (N.sup..OMEGA.)-NH.sub.2 group at this position may also be useful as an attachment site.
Hechter et. al. (J. Biol. Chem. 253: 3230-3236, 1978) has reported on the use of [8-Lys-N.sup..epsilon. -(Gly)]-vasopressin in a structure-activity study of adenyl cyclase activation in renal homogenate material. Bippi, "Peptidsynthesin am Lysin-Vasopressin", Diplomarbeit, University of Hohenheim, Germany, published April, 1980 has prepared similar peptides, i.e., [8-Lys-N.sup..epsilon. -(X)]-vasopressin wherein X is Gly or Phe. A hormonogen of this type, however, would be expected to be metabolized and inactivated quickly since it contains a primary site for aminopeptidase action, the (N.sup..alpha.)NH.sub.2 group of the cysteine residue at position 1.
It thus would be desirable to modify vasopressin and its biologically active analogs in a similar but novel manner to prolong "release" of the active polypeptide in sufficient "yield" to permit the use of relatively smaller doses.